This invention relates to composites and more particularly, this invention relates to lignocellulosic-based composite products which are resistant to insect and fungal attack.
Due to recent changes in the species, size and quality of standing timber available for harvest throughout the world, composites of lignocellulosic materials have replaced traditional solid sawn lumber for use in many structural applications. Many of these composites are used in applications which require resistance to wood-destroying organisms such as fungi and various insects. Accordingly, this requires treatment with a wood preservative.
Traditionally, solid wood products are dipped or pressure treated with solutions of preservative chemicals. However, the nature of a composite material makes it possible to incorporate a preservative into the product during its manufacture. This decreases total production costs and yields a superior product in which the composite has a constant loading of preservative throughout its thickness.
Borates have been used as broad-spectrum wood preservatives for over 50 years. Their benefits include efficacy against most wood destroying organisms such as fungi, termites and wood-boring beetles. Coupled with their low acute mammalian toxicity and low environmental impact, their fungicidal and insecticidal properties have resulted in them being considered the wood preservative of choice for most structural or construction applications. Borates such as boric acid, borax, disodium octaborate tetrahydrate (sold as TIM-BOR(copyright) wood preservative, a product of U.S. Borax Inc.) and, more recently, zinc borate are well accepted as wood preservatives. Generally, boric acid, borax and disodium octaborate are used for treating solid, wood products by dip or pressure treatment. However, these preservatives are readily soluble in water and can be incompatible with many resin systems used in producing composite products, resulting in an adverse effect on the internal bond strength of the resultant composites and poor mechanical strength. Anhydrous borax and zinc borate have been used successfully at relatively low levels with some resin systems, such as the phenol-formaldehyde resins, to produce composites with acceptable internal bond strength. See Knudson et al., U.S. Pat. No. 4,879,083. Although the low solubility borates of Knudson et al, especially zinc borate, have been used successfully to treat wood composites such as oriented strand board (OSB), fiberboard, waferboard and particleboard, they suffer from several problems in actual commercial use. For example, in working with composites containing zinc borate, metal tools, such as saws, grinders and similar cutting tools may suffer significant wear and premature failure due to the borate""s hardness. Also, the disposal of treated wood products by combustion can lead to problems in operating performance and maintenance of furnaces. It has also been found that particulate zinc borate used to treat wood composites has poor bulk flow properties which can cause difficulties in the wood composite manufacturing process.
The increased demand for treated wood composite products has resulted in a large volume utilization of borates in high capacity wood composite manufacture. Due to the very high volume throughput of commercial wood composite manufacturing facilities combined with the practice that waste wood is utilized as an energy source for wood particle drying as part of the process, an excessive build up of glassy borate deposits can occur within the furnaces. This will reduce the operating performance of the furnace as well as corrode the refractories of the furnace. In addition, the glassy borate deposits can be very difficult to remove from the furnace. See Daniels and Krapas, xe2x80x9cCombustion Characteristics of Zinc Borate-Impregnated OSB Wood Waste in an Atmospheric Fluidized Bed,xe2x80x9d 32nd International Particleboard/Composite Materials Symposium Proceedings, Mar. 3-Apr. 2, 1998, page 167 (1998).
This invention provides composites made from wood and other lignocellulosic materials which are resistant to attack by wood destroying organisms such as fungi and insects, have excellent internal bonding strength and may readily be cut, sawn and machined without excessive wear to the tools. Further, trimmings and other waste from manufacture and use of the treated composites may be disposed of by combustion without significant problems such as clogging and deterioration of the furnaces.
According to this invention, a pesticidal amount of a calcium borate is incorporated prior to forming a lignocellulosic-based composite, thereby producing composites which are resistant to insect and fungal attack.
The lignocellulosic-based composites of this invention are produced by well known procedures by combining particles of the lignocellulosic material with an adhesive binder and forming the composite, generally with heat and pressure. The calcium borate is incorporated, such as by adding to the lignocellulosic particles and/or binder, prior to forming the composite. The calcium borates are considered to have a low impact on the environment, with low mammalian toxicity, resulting in relatively safe use and disposal. They are effective fungicidal and insecticidal compounds that are relatively inexpensive, easy to store, handle and use. For example, the calcium borates have much better flowability than many other similar borates. Further, the calcium borates have some water solubility, providing rapid and continuing pesticidal activity in composites subject to exposure to low moisture environments in uses such as structural siding.
Lignocellulosic-based composites are formed from small fractions of cellulosic material, which are bonded with an adhesive binder, generally with heat and under pressure. The method of forming cellulosic-based composites is well known and has resulted in many products, including particleboard, oriented strand board (OSB), waferboard, fiberboard (including medium-density and high-density fiberboard), parallel strand lumber (PSL), laminated strand lumber (LSL), laminated veneer lumber (LVL), and similar products. Examples of suitable cellulosic materials include wood, straw (including rice, wheat and barley), flax, hemp and bagasse. The small fractions of cellulosic material can be in any processed form such as chips, flakes, fibers, strands, wafers, trim, shavings, sawdust, straw, stalks and shives.
The methods for manufacturing composites are well known and the specific procedure will be dependent on the cellulosic raw material and the type of composite desired. However, generally the cellulosic material is processed into fractions or particles of appropriate size, which may be called a furnish, mixed with an adhesive binder and the resultant mixture is formed into the desired configuration such as a mat, and then formed, usually under pressure and with heat, into the final product. The process could be considered an essentially dry process; that is, generally, no water is added to form a slurry of the materials (other than any water that may be used as a carrier for liquid resins).
The binder is preferably an adhesive resin which is cured with heat to give a strong bond between the cellulosic particles or fractions and provide structural composites with high mechanical strength. Such heat-cured adhesive resins are well known and include the formaldehyde- and isocyanate-based resins. Phenol-formaldehyde, phenol-resorcinol-formaldehyde, urea-formaldehyde, melamine-urea-formaldehyde and diphenylmethanediisocyanate are examples of suitable heat-cured resins in current use. The preferred levels of binder can typically range from about 1.5% to about 15%, but may be as low as 0.5% or as high as 25%for some composites, depending on a variety of constraints such as the particle size of the furnish and the strength and durability required of the finished wood composite. For example, structural quality OSB would typically contain between about 1.5% and 7% binder, whereas structural quality particle board may require up to 15 to 20% binder or more and medium density fiberboard (MDF) with low strength and durability requirements, such as pegboard, may contain less than 1%. Unlike many borates that have been used in the past to preserve cellulosic-based composites, the calcium borates of the present invention may be used successfully, without adverse effect on the binder or on the mechanical strength of the composite product.
The calcium borates which can be used in the method of this invention may be any of the borate compounds containing calcium, boron and oxygen. Optionally, other metallic elements, such as magnesium and sodium, may also be a part of the calcium borate molecule, i.e. calcium-sodium borates and calcium-magnesium borates. The preferred calcium borates are the calcium polytriborates, having a CaO:B2O3 ratio of 2:3, and calcium hexaborates, having a CaO:B2O3 ratio of 1:3, with the most preferred being the calcium polytriborates. Such calcium polytriborates may be synthetically produced or may be a naturally occurring borate, such as inyonite, meyerhofferite and colemanite. Examples of suitable calcium hexaborates include nobleite and gowerite. Calcium-sodium borates and calcium-magnesium borates include probertite, ulexite and hydroboracite.
The particle size of the calcium borate is not critical, but should obviously be of a size that can be readily dispersed throughout the composite product. Generally, a mean particle size of as large as about 500 microns and as small as about 1 micron may be used, but for best results, it is preferred that the particle size be in the range of from about 150 microns to about 10 microns.
The amount of calcium borate incorporated in the composite is a pesticidal amount; that is, an amount sufficient to control or kill fungi and/or insects that destroy wood and similar cellulosic-based composites products. Generally, a range of from about 0.1 to about 4 percent by weight of calcium borate, based on the composite product is used to control pests. The amount used will depend on the target pests, desired performance longevity and the expected level of precipitation exposure. Preferably, from about 0.5 to about 2 percent is used for optimum performance against both decay fungi and termites.
The calcium borate may be incorporated in the composite in any manner that will result in dispersion throughout the final product. In the case of wood-based composites, it may be mixed with the wood particles, or furnish, prior to mixing with the resin or it may be added to the resin or wood-resin mixture and then formed into a mat for pressing, heating and curing to produce the final composite. Preferably, the calcium borate is evenly distributed on wood particles such as chips or strands in order to ensure maximum contact between the wood particles and the preservative, then the resin is applied and the wood furnish is spread evenly onto plates or an endless belt (conveyor belt), forming a mat to be pressed into its final thickness. Heat is applied to cure the resin and form the final composite product. The wood furnish may contain optional amounts of additives, such as slack wax or flow agents, if desired, to aid in processing or performance, but are not essential.